Remove unneeded unlock statement (#5809)

Summary:
The dtor will automatically do unlock
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5809

Differential Revision: D17453694

Pulled By: ltamasi

fbshipit-source-id: 5348bff8e6a620a05ff639a5454e8d82ae98a22d

util/threadpool_imp.cc

@@ -313,9 +313,8 @@ void ThreadPoolImpl::Impl::BGThreadWrapper(void* arg) {
 
 void ThreadPoolImpl::Impl::SetBackgroundThreadsInternal(int num,
   bool allow_reduce) {
-  std::unique_lock<std::mutex> lock(mu_);
+  std::lock_guard<std::mutex> lock(mu_);
   if (exit_all_threads_) {
-    lock.unlock();
     return;
   }
   if (num > total_threads_limit_ ||